In the manufacture of paper, mineral fillers are commonly added. Whilst this may in some circumstances reduce the mechanical strength of the paper, i.e., relative to a paper made purely from a fibrous pulp, this is tolerated because the mechanical strength (albeit reduced) is still acceptable and there is a cost, quality and environmental benefit in being able to reduce the amount of fibre in the paper. A common property for assessing mechanical strength of paper is paper burst strength. Typically, a paper made purely from a fibrous pulp will have a higher paper burst strength than a comparable paper in which a portion of the fibrous pulp has been replaced by a mineral filler. The paper burst strength of the filled paper can be expressed as a percentage of the paper burst strength of the unfilled paper.
WO-A-2010/131016 discloses a process for preparing microfibrillated cellulose comprising microfibrillating, e.g., by grinding, a fibrous material comprising cellulose, optionally in the presence of grinding medium and inorganic particulate material. When used as a filler in paper, for example, as a replacement or partial replacement for a conventional mineral filler, the microfibrillated cellulose obtained by said process, optionally in combination with inorganic particulate material, was unexpectedly found to improve the burst strength properties of the paper. That is, relative to a paper filled with exclusively mineral filler, paper filled with the microfibrillated cellulose was found to have improved burst strength. In other words, the microfibrillated cellulose filler was found to have paper burst strength enhancing attributes. In one particularly advantageous embodiment of that invention, the fibrous material comprising cellulose was ground in the presence of a grinding medium, optionally in combination with inorganic particulate material, to obtain microfibrillated cellulose having a fibre steepness of from 20 to about 50.
Whilst the microfibrillated cellulose obtainable by the processes described in WO-A-2010/131016 has been shown to have advantageous paper burst strength enhancing attributes, it would be desirable to be able to modify, for example, further improve, one or more paper property enhancing attributes of microfibrillated cellulose, for example, the paper burst strength enhancing attributes of microfibrillated cellulose.